An internal combustion engine may include one or more turbochargers for compressing a fluid which is supplied to one or more combustion chambers within corresponding combustion cylinders. Each turbocharger typically includes a turbine driven by exhaust gases of the engine and a compressor which is driven by the turbine. The compressor receives the fluid to be compressed and supplies the fluid to the combustion chambers. The fluid which is compressed by the compressor may be in the form of combustion air or a fuel and air mixture.
The operating behavior of a compressor within a turbocharger may be graphically illustrated by a "compressor map" associated with the turbocharger in which the pressure ratio (compression outlet pressure divided by the inlet pressure) is plotted on the vertical axes and the flow is plotted on the horizontal axes. In general, the operating behavior of a compressor wheel is limited on the left side of the compressor map by a "surge line" and on the right side of the compressor map by a "choke line". The surge line basically represents "stalling" of the air flow at the compressor inlet. With too small a volume flow and too high a pressure ratio, the flow will separate from the suction side of the blades on the compressor wheel, with the result that the discharge process is interrupted. The air flow through the compressor is reversed until a stable pressure ratio by positive volumetric flow rate is established, the pressure builds up again and the cycle repeats. This flow instability continues at a substantially fixed frequency and the resulting behavior is known as "surging". The choke line represents the maximum centrifugal compressor volumetric flow rate, which is limited for instance by the cross-section at the compressor inlet. When the flow rate at the compressor inlet on other location reaches sonic velocity, no further flow rate increase is possible and choking results. Both surge and choking of a compressor should be avoided.
U.S. Pat. No. 3,044,683 (Woollenweber) discloses a fluid passage extending from the high pressure side of the compressor to the inlet side of a turbine. A spring loaded valve is disposed within the fluid passage and opens upon a high pressure condition within the compressor. The spring loaded valve thus merely acts to bypass some of the high pressure gas on an over pressure condition to the turbine of the turbocharger.
U.S. Pat. No. 5,724,813 (Fenelon et al.) assigned to the assignee of the present invention, discloses a turbocharger having a single stage compressor. A portion of the compressed gas from the single stage compressor may be recirculated to the outlet side of the turbine using controllably actuated valves. The control scheme utilizes only a single stage compressor.
The present invention is directed to overcoming one or more of the problems as set forth above.